1. Field of the Invention
This invention relates to motor driven pumps.
2. Description of Related Art
The usual arrangement when a motor drives a pump is to have the pump inline with the motor unit, as detailed, for example in DE-A-4 123 384. Unfortunately this means that the total length of the unit is at least equal to the length of the motor plus the length of the pump, and the combination may often be too long to use in confined spaces. Also the amount of materials and components needed are no less than for manufacturing the two items separately.
In order to reduce the length of pump-motor combinations two types of solution have been devised. The first is to use the rotor of the motor as the rotor of the pump, for example by threading the rotor with machined helical feed channels as in DE-A-3-937 345. This reduces the size of the pump and is a simple design requiring few components, but the pressures achieved by this pump will be very low, merely one or two bar gauge, and there is no possibility of using a more complex design of pump, for example a multiple rotor screw pump, where a driven power rotor acts in combination with one or more ancillary rotors. The design of a multiple rotor screw pump is detailed in EP-A-0 736 667.
The second previously proposed design overcomes this problem to some extent. The arrangement uses a screw pump where the central pump rotor does not turn while the ancillary screws or rotors turn with the motor rotor. This type of pump is illustrated in DE-A-3 701 586. This type of pump enables high pressures to be reached, but still contains some significant disadvantages. Two housings are requiredxe2x80x94one for the pump unit and a second for the stator assembly, which increases the bulk and the cost of the unit. The unit is also disproportionately elongate.
The present invention seeks to reduce the size and components of the motor driven pump. The invention provides a motor driven pump comprising: a stator; a rotor, coaxial with the stator, operable to rotate around the outside of the stator and a pump mechanism driven by the rotor and disposed at least partly inside the stator.
This invention provides a number of advantages compared to the prior art. In particular the stator may form part of the pump housing, reducing the number of components and the time consuming assembly required. The pumped fluid passes inside the stator and so the fluid flow through the pump may be used to cool the stator thus increasing the efficiency of the motor driven pump. This may be most effectively done when the pump housing abuts the stator, leading to efficient conduction of heat to the fluid flowing through the pump. The integration of the pump within the motor means that the mechanical and hydraulic components of the pump are acoustically shielded leading to low levels of pump audible noise. The unit can also be made to be more compact than an equivalent motor inline with a pump which means that it has many applications in confined spaces, for example under a car bonnet as part of the power steering system.
Furthermore it is possible to use the power rotor or the ancillary rotors of the pump as a hydraulic bearing for the rotor. This reduces the number of components necessary, reducing the cost and assembly time of the unit. One further bearing may function as the main bearing for both the motor rotor and the pump to ensure that there is no clash with the stator and no audible noise is caused by the vibration of the rotor. This provides an inherent advantage over the previous designs where the motor rotor was an integral part of the pump.
Lastly the system housing may contain the reservoir of fluid necessary for the running of the pump, further reducing the noise and vibration of the unit.